BFD for OSPF Session Creation and Deletion

Sessions are created and deleted in various scenarios.

All Open Shortest Path First (OSPF) neighbors are associated with an interface. When BFD is enabled on an interface and at the global level, BFD sessions are created for all OSPF neighbors that are in greater than the 2-Way state.

Each interface can have its own BFD timers. OSPF does not influence the BFD timer value for the session. The BFD module uses the configured BFD value or the default value when creating the session. A single-hop session is created for OSPF neighbors.

Note

When BFD for an OSPF session is configured, the normal OSPF hello mechanism is not disabled.
OSPF neighbor sessions do not flap when BFD is enabled or disabled on the interface where the OSPF session is associated.
OSPF assumes full connectivity between all systems on multi-access media such as LANs. If BFD is running on only a subset of systems on such a network, the assumptions of the control protocol may be violated, with unpredictable results.

BFD for OSPF sessions are deleted in the following scenarios.

An OSPF neighbor session moves to a state below 2-Way. OSPF triggers a BFD session deletion setting the reason as Path Down. When BFD receives this notification, it deletes the corresponding session. The remote BFD neighbor detects this as a detection timer expiration and propagates this session down to OSPF to terminate the session.
OSPF is disabled on an interface. All BFD sessions associated with each neighbor are deleted.
BFD is administratively disabled on an interface. BFD reports this event as a port change notification to OSPF. Upon receipt of this notification, a BFD session deletion for each neighbor is sent and BFD removes these sessions if no other client is using the same sessions. BFD communicates this change to the remote peer. When the remote BFD peer receives this Remote Admin Down event, it propagates this event to the OSPF protocol. OSPF does not provide any action for this event.